Synthesis of the C/D/E and A/B Rings of Xestobergsterol-(A)

Scalable Synthesis of an Acid Stable Analogue of Hippuristanol

Hippuristanol is a marine derived steroidal natural product with promising anticancer activity. However, instability at low pH has precluded its development as an efficient therapy. We addressed this limitation by replacing one of the oxygen atoms of the spiroketal moiety with a carbon atom. Key steps in the synthesis include a Meyer-Schuster/Nazarov cascade, a hypoiodite mediated oxyfunctionalization, and the late-stage installation of a hydroxyl group on the C-ring of the steroid.

Novel d-Annulated Pentacyclic Steroids: Regioselective Synthesis and Biological Evaluation in Breast Cancer Cells

The acid-catalyzed cyclization of benzylidenes based on 16-dehydropregnenolone acetate (16-DPA) was studied. It was found that these compounds readily undergo regioselective interrupted Nazarov cyclization with trapping chloride ion and an efficient method of the synthesis of d-annulated pentacyclic steroids based on this reaction was proposed. The structures of the synthesized pentacyclic steroids were determined by NMR and X-ray diffraction. It was found that the reaction affords a single diastereomer, but the latter can crystallize as two conformers depending on the structure. Antiproliferative activity of synthesized compounds was evaluated against two breast cancer cell lines: MCF-7 and MDA-MB-231. All tested compounds showed relatively high antiproliferative activity. The synthetic potential of the protocol developed was illustrated by the gram-scale experiment.

Formation of 5α-dihydrotestosterone from 5α-androstane-3α,17β-diol in prostate cancer LAPC-4 cells - Identifying inhibitors of non-classical pathways producing the most potent androgen

5α-Dihydrotestosterone (5α-DHT) possesses a great affinity for the androgen receptor (AR), and its binding to AR promotes the proliferation of prostate cancer (PC) cells in androgen-dependent PC. Primarily synthesized from testosterone (T) in testis, 5α-DHT could also be produced from 5α-androstane-3α,17β-diol (3α-diol), an almost inactive androgen, following non-classical pathways. We reported the chemical synthesis of non-commercially available [4-¹⁴C]-3α-diol from [4-¹⁴C]-T, and the development of a biological assay to identify inhibitors of the 5α-DHT formation from radiolabeled 3α-diol in LAPC-4 cell PC model. We measured the inhibitory potency of 5α-androstane derivatives against the formation of 5α-DHT, and inhibition curves were obtained for the most potent compounds (IC₅₀ = 1.2-14.1 μM). The most potent inhibitor 25 (IC₅₀ = 1.2 μM) possesses a 4-(4-CF₃-3-CH₃O-benzyl)piperazinyl methyl side chain at C3β and 17β-OH/17α-CCH functionalities at C17 of a 5α-androstane core.

Efficient construction of bioactive trans-5A5B6C spirolactones via bicyclo[4.3.0] α-hydroxy ketones

The intricate 5_A5_B6_C-ring fusion topologies of tricyclic spiranoid β-hydroxybutyrolactones with environmentally benign characteristics and moderate biological activities were synthesized through lactonization of the key intermediate trans-α-hydroxyindenones with malonates by an efficient and convenient short synthetic procedure.

N-Hydroxyphthalimide catalyzed allylic oxidation of steroids with t-butyl hydroperoxide

A new and optimized procedure for the allylic oxidation of Δ(5)-steroids with t-butyl hydroperoxide in the presence of catalytic amounts of N-hydroxyphthalimide (NHPI) under mild conditions was developed, showing excellent regioselectivity and chemoselectivity (functional group compatibility). It was found that Co(OAc)2 could enhance the catalytic ability of NHPI resulting in better yields and shorter reaction times. The reaction mechanism and the scope of the reaction with a variety of Δ(5)-steroidal substrates were also investigated.

Regioselective cleavage of 22-oxo-23-spiroketals. Novel cholestanic frameworks with pyranone and cyclopentenone E rings on the side chain

The regioselective opening of the F ring of 22-oxo-23-spiroketals using a saturated solution of HCl in acetic anhydride yielded novel cholestanic frameworks with pyranone or cyclopentenone E rings. The structures of the new derivatives of sarsasapogenin, diosgenin and hecogenin thus obtained were established using one and two dimensional (1)H, (13)C experiments (DEPT, COSY, HETCOR, HMBC, ROESY, and NOESY). The X-ray analysis for compound 11b confirmed the 23R configuration for the new stereogenic center.

Synthesis of 5(6)-dihydro-OSW-1 analogs bearing three kinds of disaccharides linking at 15-hydroxy and their antitumor activities

In order to study the SAR of 5(6)-dihydro-OSW-1, eight 15(α)β-O-glycosyl analogs (26-33) carrying three kinds of disaccharides including [β-d-Xylp-(1-3)-α-l-Arap], [β-d-Xylp-(1-4)-α-l-Arap] and [α-l-Rhap-(1-2)-(α)β-d-Glcp] were designed and synthesized. Their in vitro antitumor activities were tested by the standard MTT assay which disclosed that compound 33 (IC(50)=0.28-0.52 μM) showed potential antitumor activities.

Applications of total synthesis to problems in neurodegeneration: Fascinating chemistry along the way

The possibility for the application of organic synthesis to the discovery of new agents in combating neurodegenerative disorders is described. Our focus has been on agents derived from natural-product leads and natural products themselves prepared through total synthesis. Herein, we describe some of the chemistry as well as interesting observations made along the way.

The synthesis and preliminary biological evaluation of a novel steroid with neurotrophic activity: NGA0187

[structure, reaction: see text] A full account of the total synthesis of neurotrophic compound NGA0187 is provided. A key feature of the synthesis involved the direct selective oxidation of 6alpha,7beta-diol to install the unusual 6,7-ketol moiety and stereoselective conjugate addition of vinyl cuprate to enone. A preliminary evaluation of its ability to stimulate the neurite outgrowth was also evaluated with PC12 cell.

Cytochrome P450 3A4-catalyzed Testosterone 6β-Hydroxylation Stereochemistry, Kinetic Deuterium Isotope Effects, and Rate-limiting Steps

Testosterone 6beta-hydroxylation is a prototypic reaction of cytochrome P450 (P450) 3A4, the major human P450. Biomimetic reactions produced a variety of testosterone oxidation products with 6beta-hydroxylation being only a minor reaction, indicating that P450 3A4 has considerable control over the course of steroid hydroxylation because 6beta-hydroxylation is not dominant in a thermodynamically controlled oxidation of the substrate. Several isotopically labeled testosterone substrates were prepared and used to probe the catalytic mechanism of P450 3A4: (i) 2,2,4,6,6-(2)H(5); (ii) 6,6-(2)H(2); (iii) 6alpha-(2)H; (iv) 6beta-(2)H; and (v) 6beta-(3)H testosterone. Only the 6beta-hydrogen was removed by P450 3A4 and not the 6alpha, indicating that P450 3A4 abstracts hydrogen and rebounds oxygen only at the beta face. Analysis of the rates of hydroxylation of 6beta-(1)H-, 6beta-(2)H-, and 6beta-(3)H-labeled testosterone and application of the Northrop method yielded an apparent intrinsic kinetic deuterium isotope effect ((D)k) of 15. The deuterium isotope effects on k(cat) and k(cat)/K(m) in non-competitive reactions were only 2-3. Some "switching" to other hydroxylations occurred because of 6beta-(2)H substitution. The high (D)k value is consistent with an initial hydrogen atom abstraction reaction. Attenuation of the high (D)k in the non-competitive experiments implies that C-H bond breaking is not a dominant rate-limiting step. Considerable attenuation of a high (D)k value was also seen with a slower P450 3A4 reaction, the O-dealkylation of 7-benzyloxyquinoline. Thus P450 3A4 is an enzyme with regioselective flexibility but also considerable regioselectivity and stereoselectivity in product formation, not necessarily dominated by the ease of C-H bond breaking.

Unexpected endo selectivity of conjugate nucleophilic addition to an arene-Cr(CO)3 complex: enantioselective synthesis of the diterpene 11-epi-helioporin B

[reaction: see text]. Starting from a nonracemic planar-chiral arene tricarbonyl chromium complex, an epimer of the bioactive marine diterpene helioporin B was synthesized in a highly stereoselective fashion. The stereostructure of the product (11-epi-helioporin B) corresponds to that of other serrulatane-type natural products. In a key step of the synthesis, 2-lithioacetonitrile undergoes conjugate nucleophilic addition to an unsaturated complex. Remarkably, this addition occurs in an endo mode, i.e., from the complexed face of the pi-ligand.

Toward the total synthesis of variecolin

[reaction: see text] An annulative approach toward the total synthesis of the sesterterpenoid variecolin (1) is presented. Synthesis of the key hemiketal, containing the core ABC ring skeleton, has been achieved on a model system by an expeditious route utilizing samarium(II) iodide. Furthermore, enantioselective syntheses of component fragments for the total synthesis have been developed.

First total synthesis of xestobergsterol A and active structural analogues of the xestobergsterols

[formula: see text] A novel pentacyclic polyhydroxylated sterol, xestobergsterol A (1a), has been synthesized in 24 steps and in good overall yield from stigmasterol 17. The key steps of the synthesis are the Breslow remote functionalization of the polyoxygenated steroid derived from 25 and the base-catalyzed epimerization-aldol condensation of the dione derived from 27.